In recent years, with the development of portable electronic devices such as cellular phones and laptop personal computers, the demand for secondary batteries has been increased as cordless power sources for these electronic devices. Above all, the development of rechargeable lithium ion secondary batteries which are high in energy density have been carried out actively.
In addition, with functional increases of portable electronic devices, the power consumption has been increased significantly. In order to address the increase in power consumption, there has been a need for high-capacity lithium ion secondary batteries.
Lithium ion secondary batteries typically use a metal oxide such as a lithium cobalt oxide as a positive electrode active material, a carbon material such as graphite as a negative electrode active material, and lithium hexafluorophosphate dissolved in an organic solvent, that is, an organic solvent based electrolyte solution as an electrolyte. In the case of the thus configured batteries, there have been attempts to increase the internal energy by increasing the amounts of the active materials, and improve the output current by further increasing the energy density. In addition, the batteries are also expected to be increased in size, and mounted on cars.
However, in the above configured lithium ion secondary batteries, the organic solvent used for the electrolyte is a combustible substance, and the batteries thus have a risk such as battery ignition. For this reason, it has been desired to further enhance the safety of the batteries.
Therefore, one measure to enhance the safety of the lithium ion secondary batteries is to use a solid electrolyte as the electrolyte, in place of the organic solvent based electrolyte solution. It has been under consideration to apply, as the solid electrolyte, an organic solvent based material such as polymers and gels and an inorganic material such as glass and ceramics. Above all, all solid state secondary batteries have been proposed, and attracting attention, which use, as a solid electrolyte, an inorganic material containing incombustible glass or ceramic as its main constituent.
For example, Japanese Patent Application Laid-Open No. 2003-68361 (hereinafter, referred to as Patent Document 1) discloses the configuration of an all solid state lithium secondary battery including an incombustible solid electrolyte. In this all solid state lithium secondary battery, the solid electrolyte includes a sulfide as a fundamental composition, which is a material composed of a lithium sulfide and a phosphorus sulfide, or a material mainly containing a lithium sulfide and a phosphorus sulfide, containing no transition metal element, and containing no silicon or germanium; a negative electrode active material is a carbon material, or a material of lithium ions inserted between layers of a carbon material; and a lithium cobalt oxide, a lithium nickel oxide, a lithium manganese oxide, or the like is used as a positive electrode active material. In addition, Patent Document 1 discloses that battery characteristics vary greatly depending on the type of the solid electrolyte when graphite is used as the negative electrode active material, and that the selection of a lithium ion conducting solid electrolyte is important in order to prepare an all solid state lithium secondary battery which has excellent performance. Based on this consideration, it is disclosed that the use of a sulfide containing no silicon or germanium as the solid electrolyte makes it possible to increase the energy density of the all solid state lithium secondary battery.
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-68361